It is well known that aqueous solution polymerization of water-soluble ethylene-based unsaturated monomer gives a polymer in a hydrate gel form (hydrate gel-form polymer), which serves as a water-absorbing polymer. The hydrate gel-form polymer is a gel-form material being semi-solid and highly elastic. In many cases, the hydrate gel-from polymer, which is rarely used without being further processed, is crushed into small pieces (crushing product (fine particles of gel), and then dried and pulverized into particles. The thus dried and pulverized crushing product (particles) of the hydrate gel-form polymer is used as a water-absorbing resin, that is, an absorbent (in other words, gelling agent, solidifying agent) for absorbing an aqueous liquid.
In many cases, the water-absorbing resin is produced in the form of particles (especially as dried particles). In some cases, cross-linking density in a vicinity of surfaces of the water-absorbing resin particles (water-absorbing resin particles) is increased by subjecting the surfaces of the particles to secondary cross-linking treatment with a surface cross-linking agent. This secondary-liking treatment is called as surface cross-linking or surface treatment (for example, see References 1, 3, and 9). The surface treatment further improves water absorbency against pressure, liquid permeability, and water absorbency of the water-absorbing resin particles.
In order to obtain final particles of the water-absorbing resin or a final water-absorbing resin material in a particle-shape, the step of granulating, the step of adding (mixing in) an additive, and/or the like step is performed if necessary (for example, References 1 and 2) after the surface treatment. In some cases, the step of performing a particle-size regulating treatment (that is, pulverizing and/or classifying the particles) is necessary.
In case where the step of performing the surface treatment is performed, water, an aqueous solution, or the like is evaporated off by heating after the surface treatment in which the water, aqueous solution, or the like is used. The evaporation would possibly cause agglomeration of the water-absorbing resin particles. Moreover, the addition of the additive would possibly causes agglomeration of the water-absorbing resin particles with particles of the additive, agglomeration of the particles with each other, or agglomeration of the particles of the additive. Further, granulation is a treatment to intentionally agglomerate (bind the particles) the water-absorbing resin particles by using a binder such as water or the like.
The agglomeration of the water-absorbing resin particles not only deteriorates the particle-shape water-absorbing resin material (particle shape water absorbing resin composition material) in terms of its properties and the like, but also causes the particle-shape water-absorbing resin material to be unacceptable as a final product. Therefore, it is necessary to subject, to the particle-size regulating treatment, the particle-shape water-absorbing resin material that contains agglomerated particles (agglomerations) and/or large-size particles (The agglomerated particles and the large-size particles have a weight-average diameter of 850 μm or larger, for example). Note that the particle-size regulating treatment is a treatment to give the particle-shape water-absorbing resin material a uniform shape and/or a uniform diameter (for example, the particle-shape water-absorbing resin material is given a weight-average diameter of less than 850 μm to pass through a sieve of 850 μm mesh). Generally, the particle-size regulating treatment is carried out by pulverizing and/or classifying the particles or the agglomerations of the particles.
Moreover, there is known arts in which an additive such as an inorganic powder is added to the surface-treated water-absorbing resin particles, for further improving the properties of the water-absorbing resin particles (for example, see References 1, 3 to 8, and 12). It is preferable that the particles of the additive and the water-absorbing resin particles be mixed uniformly in the particle-shape water-absorbing resin material. In order to mix them uniformly, it is suggested adding, for example, silica (particulates of silicone oxide) or the like in the water-absorbing resin particles, so as to facilitate stirring. Many methods and apparatuses for mixing the additive have been suggested, but there has been no method or apparatus that performs the mixing and the particle-size regulating treatment of the particles in combination. According to the method in which the mixing and the particle-size regulating treatment of the particles are performed in combination, silica or the like is added into the water-absorbing resin particles after the particle-size regulating treatment. Therefore, it is expected that it is possible to uniformly mix the water-absorbing resin particles whose particle size has been regulated by the particle-size regulating treatment.
Moreover, generally, the water-absorbing resin particles are produced by continuous process in which a number of steps are continuously performed (for example, see References 10 to 11).
Reference 1: Japanese Publication of Patent Application, Tokukaisho, publication No. 61-97333 (published on May 15, 1986) which corresponds to the specification of the U.S. Pat. No. 4,734,478;                Reference 2: the Specification of U.S. Pat. No. 5,369,148;        Reference 3: the Specification of U.S. Pat. No. 5,981,070;        Reference 4: the Specification of U.S. Pat. No. 5,797,893;        Reference 5: the Specification of U.S. Pat. No. 5,985,944;        Reference 6: the Specification of U.S. Pat. No. 5,229,488;        Reference 7: the Specification of U.S. Pat. No. 6,124,391;        Reference 8: the Specification of published U.S. Patent application No. 2002/0128618 A1;        Reference 9: the Specification of U.S. Pat. No. 5,610,220;        Reference 10: the Specification of U.S. Pat. No. 6,727,345;        Reference 11: the Specification of U.S. Pat. No. 6,716,894; and        Reference 12: the Specification of U.S. Pat. No. 6,720,073.        
However, it has found out that, even if the properties of the water-absorbing resin particles are improved by the surface treatment, the addition of the additive results in deterioration of the properties of the particle-shape water-absorbing resin material or failure in sufficiently improving the properties. As a result of intensive studies to find out the reason for the deterioration of the properties of the resultant particle-shape water-absorbing resin material in the method in which the additive is added to the surface-treated water-absorbing resin particles, attention is focused on the particle-size regulating treatment, which has been carried out conventionally by necessity.
The step of performing the particle-size regulating treatment for the particles, which include the agglomerated particles (agglomeration), of water-absorbing resin is a step for converting the agglomerated water-absorbing resin particles into primary particles thereof substantially or mostly. Meanwhile, the step of mixing is a step for mechanically stirring the particles of the water-absorbing resin. (Note that the agglomerations are agglomerated primary particles and have interfaces between the particles, and the primary particles are non-granulated or non-agglomerated single particles, which are obtained by drying a polymer and then, if necessary pulverizing and or classifying the thus dried polymer.) The studies demonstrate that both the step of performing the particle-size regulating treatment and the step of mixing give damages on the surfaces of the water-absorbing resin particles and/or the additive. Even if the water-absorbing resin particles have been subjected to the surface treatment, the damages on the surfaces would possibly reduce effects of the surface treatment in the arrangement in which the mixing is followed by the particle-size regulating treatment.
In other words, it was found that, if the step of mixing the additive such as silica or the like is carried out after the step of performing the particle-size regulating treatment of the water-absorbing resin particles, the surfaces of the water-absorbing resin particles are damaged not only by the particle-size regulating treatment but also the mechanical energy applied thereon in mixing. Whereby, the water-absorbing resin particles as a final product has poor properties undesirably.
On the other hand, it is an option to add silica to the water-absorbing resin particles before subjecting the water-absorbing resin particles to the particle-size regulating treatment. The addition of silica makes the stirring more effective. In this arrangement, however, the uniform mixing cannot be attained. Specifically speaking, because silica is added to the water-absorbing resin particles before the particle-size regulating treatment, silica is not uniformly mixed with the water-absorbing resin particles whose particle size has been regulated by the particle-size regulating treatment, even if it is uniformly mixed with the water-absorbing resin particles before the particle-size regulating treatment. Thus, the sufficient effect of the stirring cannot be attained. Therefore, in general, it is not practical to perform the particle-size regulating treatment of the water-absorbing resin particles after adding the silica therein.
Further, an art has been suggested in which silica is added to the water-absorbing resin particles that have not been subjected to the surface treatment, and then the particles with silica are subjected to crushing (for example, see References 1 and 2). In this art, silica is added to the water-absorbing resin particles in the granulation, for the purpose of giving the water-absorbing resin particles a larger weight-average diameter. That is, this art has no technical idea to uniformly mix silica and the water-absorbing resin particles, or to give various functions to the water-absorbing resin particles. Therefore, it is impossible to apply this art to the surface-treated water-absorbing resin particles.